A gyrometer is a movement sensor which makes it possible to measure the rotation speed of the reference frame of the sensor with respect to a Galilean reference frame, about one or more axes.
Laser gyrometers, also referred to as laser gyros, for the most part use a gaseous amplifier medium which is conventionally a mixture of helium and neon. It is, however, possible to use a laser gyro with a solid-state amplifier medium, in which the gaseous amplifier medium is replaced by a solid element, for example a matrix of YAG (yttrium aluminum garnet) doped with neodymium.
The operating principle of a laser gyro is based on the Sagnac effect of a bidirectional laser ring cavity to which a rotational movement is imparted. The Sagnac effect induces a frequency difference S2 between two so-called counter-propagating optical emission modes propagating in opposite directions inside the cavity. In the solid media conventionally used, including Nd:YAG, the modes propagating in opposite directions share the same amplifier atoms. The term homogeneous gain is then used. When the two counter-propagating modes have equal or very similar frequencies, the interference signal which results therefrom is a possibly mobile standing wave. The atoms of the gain medium participate commensurately more in the stimulated emission process when they are close to an antinode of the standing wave, and commensurately less when they are close to a node. A population inversion network is then created in the gain medium, circumscribed by the standing wave. This network remains so long as the frequencies of the two counter-propagating modes are sufficiently close together. Its contrast is commensurately less when the frequency difference is large compared with the inverse of the lifetime of the excited level.
French patent application FR 2905005 (THALES) describes a laser gyro comprising at least one optical ring cavity and a solid-state amplifier medium, which are arranged so that two so-called counter-propagating optical modes can propagate in opposite directions to one another inside said optical cavity and pass through the amplifier medium. The amplifier medium is coupled to a transducer device which provides the amplifier medium with a periodic translational movement along an axis substantially parallel to the propagation direction of said optical modes.
Such a device makes it possible to modulate the longitudinal position of the active crystal around an average position, so that the atoms of the crystal are in movement with respect to the nodes and antinodes of the interference pattern formed by the two counter-propagating modes, whatever the frequency difference between these two modes. Such a device makes it possible to reduce the contrast of the gain network and therefore its detrimental effects for the gyrometry measurements, while not modifying the length of the cavity. It also makes it possible to attenuate the effects of the backscattering induced by the amplifier medium. Lastly, this device potentially constitutes a device for processing the blind zone which, depending on the case, may substitute for or be complementary to the conventional mechanical activation device.
Such a device must permit a high excitation frequency, which is necessary for use in the field of civil aviation or in a weapons system. This excitation frequency must be greater than the frequency difference which occurs between the two counter-propagating modes for an input angular frequency of the gyrometer corresponding to the intended measurement range.
One technical difficulty consists in obtaining a sufficiently high mechanical excitation frequency, of the order of a few hundreds of kHz, combined with a large oscillation amplitude of the order of a few microns, without affecting the geometrical and dimensional characteristics of the crystal forming the solid amplifier medium. The reason is that the intended excitation frequency is close to the natural vibration frequencies of the crystal or any other solid of similar dimension, of the order of a few millimeters.